Metabolism Of Propionibacteria

Although not a lactic acid bacterium, Propionibacterium freudenreichii subsp. shermanii is an important part of the thermophilic starter culture used to manufacture Swiss-type cheeses. This organism is not only responsible for producing CO2 that leads to eye or hole formation, but it also produces other compounds, including amino acids and their degradation products, that contribute to the characteristic flavor of these cheeses (Gagnaire et al., 1999).

During Swiss cheese manufacture, growth of Pro. freudenreichii subsp. shermanii does not occur until the primary lactic fermentation is completed and cheese is moved into a ''warm room'' held at 20-25°C. Although propionibac-teria can ferment lactose, essentially none is available at this time and instead lactate is the primary energy source for their growth in cheese. Fermentation of lactate yields propionate, acetate, and CO2, with a theoretical molar ratio as:

3 lactate ^ 2 propionate + 1 acetate + 1 CO2

In cheese, the actual amount of CO2 may vary either as a result of condensation reactions, cometabolism with amino acids, or strain variation. The propionate pathway consists of many reactions, and it requires several metal-containing en-

Figure 12 The propionate pathway of Propionibacteria. Only the key intermediate compounds are shown. A more complete description of the enzymes and cofactors is given by Piveteau, 1999.

zymes and vitamin cofactors (Fig. 12). Enzymes of the citric acid cycle are also required. One mole of ATP is generated per mole of lactate consumed.

Although proteolysis of casein by Pro. shermanii is limited because of low proteinase activity, it does produce several peptidases (Gagnaire et al., 1999; Langsrud et al., 1995). These peptidases are located intracellularly, and their substrates are the peptides released by starter culture proteinases and residual milk and coagulant proteinases. Although no information on peptide transport systems in propionibacteria is currently available, there is evidence that some peptidases could be released via autolysis (0stlie et al., 1995). Several peptidases have activity on proline-containing peptides, accounting for high levels of proline that accumulate in Swiss-type cheeses. In addition, metabolism of the amino acids alanine and aspartate may contribute to CO2 production (Langsrud et al., 1995).

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